Micro Electro Mechanical Systems (MEMS) often utilize micromachined structures such as beams, slabs, combs, and fingers. These structures can exhibit curvature due to internal stresses and doping gradients. The curvature can be a significant source of error in inertial sensors such as accelerometers and gyroscopes. Many desired structures have a flatness design criteria that is difficult or impossible to achieve using current processes. In particular, silicon layers heavily doped with boron can have a significant curvature when used in suspended structures.
The aforementioned structures are often made starting with a silicon wafer substrate. A boron-doped silicon epitaxial layer is then grown on the silicon wafer substrate and is subsequently patterned in the desired shape. As is further described below, the boron is used as an etch stop in later processing to allow the easy removal of the silicon substrate, leaving only the thin boron-doped epitaxial layer.
At the interface between the boron-doped epitaxial layer and the silicon substrate, the boron tends to diffuse out of the epitaxial layer and into the silicon substrate. This depletes the epitaxial layer of some boron, and enriches the silicon substrate with boron. The epitaxial layer thus often has a reduced concentration of boron near the interface, which is sometimes called the “boron tail.”
After the boron-doped silicon epitaxial layer has been grown to the desired thickness, or at some later point of processing, the silicon substrate is removed often using an etchant that is boron selective. Specifically, the etchant will etch away the silicon substrate, but not the boron-doped silicon epitaxial layer. One such etchant is a solution of ethylene diamine, pyrocatechol, and water (EDP). The etchant typically etches the silicon at a fast rate up to a certain high level boron concentration, at which point the etch rate significantly slows. This high boron concentration level is termed the etch stop level.
The boron concentration near the epitaxial layer surface having the boron tail may be lower than the etch stop level, allowing the etching to remove some of the epitaxial layer surface at a reasonable rate, stopping at the etch stop level of boron concentration beneath the initial surface. The resulting boron-doped structure, such as a beam, thus has two surfaces, the silicon side surface that has the boron tail and the air side surface that has a boron surface layer concentration substantially equal to the concentration in the bulk of the beam away from either surface. Thus, the opposing surfaces have different boron surface layer concentrations.
Boron occupies substitutional lattice sites in silicon, the boron having a Pauling's covalent radius roughly 25% smaller than that of silicon. The size difference causes the boron-doped layers to shrink relative to the undoped or lower doped layers. This size difference leads to an initial tensile stress, with higher boron concentrations leading to higher tensile stresses and lower boron concentrations leading to lower tensile stresses. After release from the substrate, the lower boron concentrations in the tail results in a relatively lower tensile stress than the tensile stress in the air side layer having a higher boron concentration. The tensile stress can transition to a compressive stress after further process steps, such as oxidation and annealing at high temperatures. Regardless of the exact mechanism, an unequal surface layer boron concentration in silicon can lead to an unequal application of stress by those layers in the structure which can lead to the cupping or out-of-plane bending and curvature of a structure where flatness is desired.
What would be desirable, therefore, is a process for reducing the unequal surface layer concentrations of boron in boron-doped silicon to produce substantially flat or planar boron-doped silicon microstructures.